Most of the devices developed to this day for the recovery of vibrational energy use a mobile mass connected to an elastic element that, once excited, begins to oscillate. This oscillation movement allows electric current to be generated in two ways:                through a piezoelectric generator (see, for example, patents FR2838885 and JP2005335692),        through induction, by coupling the oscillating mass, featured by suitable magnetic properties, with a fixed coil (see for example, U.S. Pat. No. 7,009,315).        
In U.S. Pat. No. 7,009,315, for example, vibration is applied to a copper coil and is transmitted through a spring to a magnet causing a relative movement between the copper coil and the magnet. Such a movement generates a variable magnetic field which causes electromagnetic induction that in turn induces the generation of current in the copper coil. Other examples of systems of this type are disclosed in WO2005022726, GB2311171 JP8065992.
WO2008/062377 discloses a device for converting the mechanical energy associated with the vibration generated by vibrating means to which the device is connected into electrical energy, the device comprising at least one ferromagnetic stator with a coaxial electric winding and a magnetic mass which is mobile with respect to the stator and forms a magnetic circuit with the stator and the electric coil. The magnetic mass is movable perpendicularly with respect to the axis of the coil in response to vibrations generated by the vibrating means, whereby the magnetic circuit has a correspondingly variable gap width. Elastic means are provided at the ends of stroke of the mobile magnetic mass and the coil is electrically connected to generated energy storage means through a conditioning circuit. Like for almost all analogous existing devices, the previous system described hereby uses ferromagnetic material on which the magnetic flux is closed. The circuit in ferromagnetic material increases the weight and the bulk of the device and limits the possibility of miniaturization. Moreover, the presence of a substantial volume of ferromagnetic material, with respect to the volume of the entire device, causes a decreasing of the efficiency due to magnetic hysteresis cycle of the ferromagnetic material and to energy dispersion associated with the parasitic currents induced therein.
Another electromechanical generator for converting vibrational mechanical energy into electric energy is described in GB 2425222. The generator according to this patent comprises a substantially annular high-permeability core including at least one permanent magnet, so as to define a magnetic circuit, and an electric winding around an arm thereof. The core is pivotally mounted on a support and on a diametrically opposite side it has a gap within which a body made from a highly permeable material is arranged. The body is spaced from the respective end faces of the annular core, so that an oscillation of the core around its oscillation axis generates a relative movement between the end faces of the core and the body. The variable relative position between the end faces of the core and the body produces variations in the magnetic flux in the core which induce an electric current in the winding. The passing of the magnetic flux through the body generates a force contrasting the movement generated by the vibration on the oscillating core which forces the core to bring its end faces back into alignment with the body.
The aforementioned generators have structural characteristics that limit the possibility of miniaturizing them: for example, due to their intrinsic bulk, it is not possible to make an optimal arrangement of the electronic components inside them; the presence of the ferromagnetic cores has a negative impact upon their weight, on their efficiency, as well as on their constructive simplicity.